1. Field of the Invention
The present invention relates to a magnetic carrier, a two-component developer, and a developer for replenishment which are each used for visualizing an electrostatic image by electrophotography; the present invention also relates to an image forming method in which the magnetic carrier, the two-component developer, and the developer for replenishment are used.
2. Description of the Related Art
In general electrophotographic image forming methods which have been typically employed, an electrostatic latent image is formed on an electrostatic latent image-bearing member by various techniques, and toner is attached to the electrostatic latent image to develop the electrostatic latent image. In the development of the electrostatic latent image, two-component development has been widely employed; in particular, a particulate carrier called magnetic carrier is mixed with toner for triboelectric charging of the toner with the result that the toner receives a proper amount of positive or negative charge, and the electrostatic latent image is developed with the charge that serves as driving force.
In the two-component development, the magnetic carrier can be mixed with the toner to charge and carry the toner; hence, the magnetic carrier plays a distinct role from the toner, which produces advantages such as enabling the properties of the developer to be well controlled.
In recent years, technical progress in electrophotography has generated increasingly high demands for formation of a high-definition image and stable formation of a high-quality image as well as high-speed processing and the prolonged lifetime of an apparatus. In order to satisfy such demands, improving the performance of a magnetic carrier and decreasing the specific gravity thereof have been needed.
In one of techniques which have been proposed, uneven image density in long-term use and a color change in full-color printing are reduced (Japanese Patent Laid-Open No. 4-93954). In this technique, the core of a magnetic carrier has an uneven surface profile, and the core is coated with resin such that irregularity reflecting the uneven surface profile remains on the surface of the coated core. Although such an approach satisfies the above-mentioned requirements to some extent, the magnetic carrier particles have a large specific gravity and thus apply a load onto the toner; hence, the lifetime of the developer becomes short in a recent circumstance in which high-speed printing is needed. Thus, further improvement has been needed to satisfy demands for high image quality and adaptability to a change in environment.
Under such a circumstance, other techniques have been proposed, which involve use of a magnetic carrier which includes a porous magnetic core having pores formed therein and a small specific gravity (Japanese Patent Laid-Open Nos. 2009-175666 and 2012-63571). Such techniques have prolonged the lifetime of a toner to some extent.
These days, office network has been becoming common, and service for users of copiers, such as e-maintenance, has been popular; then, such a tendency has led to a maintenance-free system. In view of introduction of a maintenance-free system, a magnetic carrier having a longer lifetime is needed, and it is therefore highly necessary to develop a magnetic carrier and two-component developer satisfying such a need as well as an image forming method in which the magnetic carrier and the two-component developer are used.